1. Field of the Invention
The present invention relates to a dither pattern generating apparatus, a dither pattern generating method, and a dither pattern generating program and, more particularly, to a dither pattern generating apparatus, a dither pattern generating method, and a dither pattern generating program used in the case of performing a binarizing process on an image by using a dither pattern.
2. Description of the Related Art
Many methods for correcting a density change accompanying deterioration with time of an output device at the time of binarizing an image and outputting the binary image by a dithering process have been proposed.
For example, Japanese Laid-Open Patent Publication No. 2001-111830 discloses a method of measuring a plurality of dither patterns which are output by a sensor and calculating a γ characteristic curve in order to generate a dither pattern having a linear characteristic by correcting a density change in an output device.
Japanese Laid-Open Patent Publication No. 09-298663 discloses a tone transforming apparatus for pre-storing original dither matrixes of a number of kinds larger than the number of tones of image data, transforming thresholds on the basis of a tone transforming characteristic which is set by the user, and storing the resultant as a dither matrix.
In the method disclosed in Japanese Laid-Open Patent Publication No. 2001-111830, a sensor value measured by a sensor in a printer is converted to an actual print density value on the basis of a fixed graph indicative of the relation between a sensor value and a print density value (hereinafter, referred to as “sensor density curve”).
In reality, however, since an actual sensor density curve gradually changes due to fluctuations in environment, deterioration in an engine, and the like (generically also simply referred to as “change in environment”), there is a problem such that the present circumstances are not accurately reflected in an approximated γ characteristic curve.
FIG. 15(A) is a diagram showing a state where a sensor of a type of sensing reflection light (when a sensor value is large, print density is low and, when a sensor value is small, print density is high) is used, and the relation between the sensor value and the print density fluctuates according to endurance (increase in the number of print sheets).
As shown in FIG. 15(A), the relation between a sensor value and print density tends to gradually fluctuate from an initial curve (solid line) (to a broken line). Therefore, as shown in FIG. 15(B), when the relation (γ characteristic curve) between the pattern No. of a dither pattern and print density is estimated on the basis of an initial sensor density curve (solid line), a problem occurs such that it is deviated from a true γ characteristic curve (broken line) estimated on the basis of the sensor density curve after fluctuation.
The problem occurs for the following reason. Since the sensor senses reflection light, when the number of flaws in the surface of a medium (for example, an intermediate transfer belt) for outputting a dither pattern increases due to deterioration, irregular reflection components on a bare surface increases and the sensor sensitivity to a highlight area tends to increase. On the other hand, printed toner covers the flaws in the bare surface, so that the sensor value for a shadow area does not largely fluctuate also after deterioration.
As a result of a change in the sensor density, as shown in FIG. 15(B), the γ characteristic curve (solid line) of which density is generally lower than actual one (broken line) is erroneously estimated, and a γ correcting process cannot be performed properly.